This invention relates to a method of injecting chemicals into a hydrocarbon reservoir for the purpose of increasing hydrocarbon recovery. More particularly, the invention pertains to the use of urea as a sacrificial agent to decrease adsorption of surfactants within hydrocarbon reservoirs.
One of the most vexing problems in the use of surfactant flooding for enhanced oil recovery is the frequent, substantial loss of surfactant due to adsorption on the formation matrix and precipitation by polyvalent cations such as calcium and magnesium. A significant percentage of surfactants are also physically entrapped within the pore spaces of the rock matrix. Of chief concern is surfactant adsorption on the formation matrix which significantly decreases surfactant flood efficiency, making it necessary to inject a greater quantity of surfactant and increasing the cost of any surfactant flood.
Additionally, most surfactants are satisfactory for surfactant flooding only if the calcium and magnesium concentrations of the formation water fall below about 500 ppm. Alkyl or alkylaryl sulfonates, the most popular type of surfactants, precipitate where divalent ion concentrations exceed about 500 ppm. Such precipitation renders the sulfonates inoperative for recovering oil and in some instances, causes formation plugging.
The main cause of surfactant loss is adsorption within the formation due to physical contact of the surfactant with the formation matrix or entrapment within pores of the matrix. Surfactant systems contacting a sandstone matrix encounter a range of adsorptive sites. Although a number of adsorptive sites in sandstone are negatively charged, there are also a number of positively charged sites. For example, the surfaces of clay platelets, which are invariably present in sandstone reservoirs, have negatively charged sites, and the edges of clay platelets frequently have positively charged sites. The presence of small amounts of limestone or dolomite in sandstone will additionally introduce positively charged absorptive sites. Of course, different types of clays will have different adsorptive characteristics.
In addition to the composition of the formation matrix, it is known that the pH of the formation brine and of injected solutions can have a marked effect on adsorption of anionic surfactants. See M. J. Rosen, Surfactants and Interfacial Phenomena, John Wiley and Sons, New York, N.Y. (1978) p. 47.
The most promising approach for reducing the amount of surfactants retained by the formation matrix has been to use sacrificial agent compounds, either in a preflush solution injected before the surfactant-containing solution, or in the surfactant solution. The compounds are sacrificial in that their loss to the formation matrix reduces the loss of the more expensive surfactants and solubilizers contained within the surfactant solutions.
Various chemicals have been employed as sacrificial agents to decrease the adsorption of surfactants or to tie up polyvalent cations and prevent them from precipitating surfactants from the flooding medium. Lignosulfonates form one class of compounds which have been found to have excellent properties as sacrificial agents. They are economically attractive because they are by-products of the pulp industry. Supply is plentiful and product costs are much less than the costs of surfactants employed in enhanced oil recovery floods. The use of various lignosulfonates has been disclosed extensively in the literature.
U.S. Pat. No. 4,147,214 discloses the use of synthetic tannin materials as sacrificial agents in surfactant floods. The synthetic tannins may be condensation products of formaldehyde and naphthalene sulfonic acids, various phenols and sulfonated phenols, diaryl sufones, urea, melamine and diamide. U.S. Pat. No. 4,493,370 discloses the use of urea as a cosurfactant with hydrocarbon sulfonates and alcohol in a surfactant system for enhanced oil recovery.